1. Field of the Invention
The present invention relates to a small-size image pickup apparatus, and specifically to an image pickup apparatus for an endoscope including an image pickup unit having a variable aperture mechanism for lenses in an objective optical system of the endoscope.
2. Description of the Related Art
Endoscopes have been widely used in the medical and other fields for some time. By inserting an elongated insertion portion or the like into a body cavity, it is possible to observe organs in the body cavity. Where necessary, various treatments can be performed by inserting treatment instruments into a treatment instrument passing channel. Endoscopes include a bending portion at a distal end side of the insertion portion, and it is possible to change an observation direction by performing operations on an operation portion.
Image pickup apparatus used in the above-described endoscope, super minisize cameras and the like have a lens configuration with a single focal distance, so-called fixed focal point lenses, due to the requirements of miniaturization. When a variable aperture is included, however, it is possible to move the focal distance towards a nearer side by reducing the size of the opening of the variable aperture.
Japanese Patent Application Laid-Open Publication No. 9-318987 discloses technology relating to an image pickup apparatus which includes a lens portion formed to have a fixed focal point, a variable aperture having a rotation member for restricting light from the lens portion, and a driving unit for driving in forwards and backwards directions with respect to the lens portion. The arrangement is such that as the rotation member is rotated by the driving unit, the lens portion is caused to move backwards and forwards.
In conventional image pickup apparatus, because a position of the focal point changed by using a variable aperture, it is difficult to secure suitable optical characteristics. To deal with this fault, the image pickup apparatus may be provided with a bifocal lens (or multiple focal lens) having the suitable optical characteristics. Alternatively, a lens can be moved to match a focal point position as in the image pickup apparatus as disclosed in Japanese Patent Application Laid-Open Publication No. 9-318987.
However, in the image pickup apparatus recorded in Japanese Patent Application Laid-Open Publication No. 9-318987, a driving unit for moving the lens is required.
Moreover, the variable aperture mechanism which forms the variable aperture has a complex structure made up of members with a low degree of strength. Consequently, to obtain suitable optical characteristics with a variable aperture mechanism installed in an image pickup unit including a bifocal lens, it is necessary to position the variable aperture mechanism with precision in relation to the bifocal lens.